KVM: x86: Replace (almost) all guest CPUID feature queries with cpu_caps

Switch all queries (except XSAVES) of guest features from guest CPUID to
guest capabilities, i.e. replace all calls to guest_cpuid_has() with calls
to guest_cpu_cap_has().

Keep guest_cpuid_has() around for XSAVES, but subsume its helper
guest_cpuid_get_register() and add a compile-time assertion to prevent
using guest_cpuid_has() for any other feature.  Add yet another comment
for XSAVE to explain why KVM is allowed to query its raw guest CPUID.

Opportunistically drop the unused guest_cpuid_clear(), as there should be
no circumstance in which KVM needs to _clear_ a guest CPUID feature now
that everything is tracked via cpu_caps.  E.g. KVM may need to _change_
a feature to emulate dynamic CPUID flags, but KVM should never need to
clear a feature in guest CPUID to prevent it from being used by the guest.

Delete the last remnants of the governed features framework, as the lone
holdout was vmx_adjust_secondary_exec_control()'s divergent behavior for
governed vs. ungoverned features.

Note, replacing guest_cpuid_has() checks with guest_cpu_cap_has() when
computing reserved CR4 bits is a nop when viewed as a whole, as KVM's
capabilities are already incorporated into the calculation, i.e. if a
feature is present in guest CPUID but unsupported by KVM, its CR4 bit
was already being marked as reserved, checking guest_cpu_cap_has() simply
double-stamps that it's a reserved bit.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20241128013424.4096668-51-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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